Influenza was little known, particularly from the bacteriologic viewpoint, before the pandemic of 1889-90. This, occurring at a time when bacteriology was making important contributions to medicine, naturally proved a fruitful field for research and resulted in the discovery by Pfeiffer of a bacillus (Bacillus influenzœ) which since then has been generally accepted as the causal organism. Many epidemics of clinical influenza have appeared since, and in some Pfeiffer's bacillus has been found, while in others a variety of organisms has been described. The idea is prevalent that every condition occurring in epidemics and giving the clinical picture of influenza is due to the influenza bacillus, and that if influenza bacilli are found in the respiratory passages in these conditions the diagnosis of influenza is justified.
One purpose of this paper is to call attention to conditions which the physician commonly calls influenza or grip, and with which